Heretofore various processes have been employed for obtaining metallic iron by the reduction of iron bearing sources such as ore. For example, U.S. Pat. No. 3,585,023 discloses a process in which iron ore is mixed with a reducing agent such as a fossil fuel and forming the mixture into agglomerates. The thus formed agglomerates are then charged to an enclosed environment which utilizes hot solid inert particles as the heating medium, said enclosed environment being maintained in the absence of oxygen. Upon completion of the desired reduction, the particles and agglomerates are separated and the reduced agglomerates are allowed to partially cool before being exposed to the atmosphere. Following this the reduced agglomerates are then recovered. Other methods utilize a heating and magnetic separation to effect a production of iron powder as well as effecting a physical separation by crushing, scalping, air cleaning, and screening followed by subjecting the ore to a high intensity induced magnetic field. U.S. Pat. No. 2,728,655 discloses a method for producing iron powder with a low silica content in which an iron containing material in finely divided form is admixed with a solid ash-containing carbonaceous reducing agent. The mixture is then heated to complete the reduction of the iron containing material followed by cooling and recovery of the iron content.
As will hereinafter be shown in greater detail, it has now been discovered that by subjecting the iron bearing source to a series of steps it is possible to obtain an iron powder which will contain less than 0.1% silica, which is considerably less than the silica content of iron which remains after the normal treatment of iron bearing sources.
This invention relates to an improved process for the recovery of metallic iron from an iron bearing source. More specifically, the invention is concerned with the process for obtaining a high purity iron powder which is substantially free of silica and other impurities, and which may then be utilized for a variety of powder metallurgy applications.
Heretofore, iron powder which is suitable for use in powder metallurgy applications was obtained from atomized iron powder which is relatively expensive to produce. Powdered iron which is substantially free of impurities such as silica, for example, iron which possesses a silica content less than about 0.1%, may be used for the production of certain-types of steel, in which the powder is directly rolled into sheet steel. In addition, another use of high purity iron powder is in molding in which the iron powder is directly pressed into different configurations for use as a particular part of various apparatuses, rather than by casting which is the usual method for obtaining parts which possess a particular configuration.
It is therefore an object of this invention to provide a process for obtaining high purity metallic iron.
A further object of this invention is to provide a process for obtaining iron powder which possesses a silica content of less than about 0.1%.
In one aspect an embodiment this invention resides in a process for the recovery of metallic iron from an iron bearing source which comprises the steps of grinding said iron bearing source, subjecting the ground iron bearing source to a caustic leach at an elevated temperature and presusre, separating the impurities from the caustic leach residue, reducing the iron in said caustic leach residue by treatment with hydrogen at an elevated temperature and pressure, and recovering the resultant metallic iron.
A specific embodiment of this invention is found in a process for the recovery of metallic iron from an iron bearing source which comprises the steps of reducing an iron concentrate by treatment with hydrogen at a temperature in the range of from about 700.degree. to about 1300.degree. F. and a pressure in the range of from about atmospheric to about 500 psi, thereafter subjecting the reduced source to at least one magnetic separation step to separate impurities such as silica therefrom, grinding the iron bearing source to a desired particle size, subjecting the ground iron bearing source to a caustic leach by treatment with sodium hydroxide at a temperature in the range of from about 300.degree. to about 400.degree. F. and a pressure in the range of from about 200 to about 400 psi to solubilize impurities such as silica, aluminum, calcium and magnesium, filtering the leach mixture, washing the residue with a water solution containing hydrochloric acid, filtering the solution, thereafter subjecting the residue to a second reduction step by treatment with hydrogen at a temperature in the range of from about 700.degree. to about 1900.degree. F., grinding the reduced iron to a particle size in the range of from about -40 mesh to about +270 mesh, thereafter subjecting the ground iron to at least one magnetic separation step to further separate any impurities which may still be present, and recovering the purified iron powder.
Other objects and embodiments will be found in the following detailed description of the present invention.
As hereinafter set forth, the present invention is concerned with an improvement in a process for obtaining high purity iron powder from an iron bearing source. By effecting the process in a series of steps hereinafter set forth, it has been found possible to obtain iron powder which contains less than about 0.1% silica, thus enabling the powder to be used for a variety of powder metallurgic purposes. Several advantages in the treatment of the iron bearing source when utilizing the various steps of the process of the present invention will become apparent when discussing the process. The iron bearing source which may be utilized as the feed stock for the present invention will comprise hematite (Fe.sub.2 O.sub.3) and magnetite (Fe.sub.3 O.sub.4) concentrates which are widely available from many sources, the most common source which is presently available being hematitic in nature. However, the concentrates usually contain a relatively large amount of silica (SiO.sub.2) as the major gangue mineral in amounts ranging from about 1 to about 7%. In addition, other impurities are also present such as minor amounts of aluminum, calcium, magnesium, etc. In order to be able to use iron powder for various metallurgical purposes, it is necessary to remove the impurities, especially silica, and reduce the concentration of the silica to less than about 0.1%. To effectively attain the desired result, the iron bearing source such as hematite must be processed through a series of steps in order to reduce the iron oxide concentrate to pure iron.
The process of the present invention is effected by subjecting an iron bearing source such as hematite or magnetite to a grinding process whereby the iron ore is reduced to a desired particle size. The particle size will be such that a major portion of the silica is liberated from the iron and therefore the desired particle size should be about -200 mesh, this size being sufficient to effectively separate the silica particles from the ore. Thereafter the ground ore is subjected to a caustic leach at an elevated temperature and pressure. The leach solution which is utilized to effect a separation of silica will comprise an aqueous sodium hydroxide solution which contains from about 100 to about 200 grams/liter of sodium hydroxide, although a lesser amount or greater amount of sodium hydroxide may be employed if so desired. The leach is effected at temperatures which may range from about 300.degree. to 400.degree. F. at an implied pressure in the range of from about 200 to about 400 psi. After treating the ore with the caustic solution for a period of time which may range from about 0.5 up to about 4 hours or more in duration, the silica particles in the leach head material will form sodium silicate of varying composition and which will be soluble in nature. The caustic treatment of the ore will result in the reduction of the silica content present in the original ore down to less than 1%. Following the treatment of the ore within the reaction parameters set forth above, the solid ore which may also contain insoluble silicates of aluminum, calcium, and magnesium, will be separated from the leach liquor by conventional means such as filtration, centrifugation, decantation, etc. The residue may then be subjected to a reducing process in which the iron is treated with hydrogen at an elevated temperature and pressure for a predetermined period of time in either a one or two step reduction process. In one embodiment the iron ore is reduced by treatment with hydrogen at a pressure in the range of atmospheric to about 500 psi in a two step process, the first step being effected at a temperature in the range of from about 700.degree. to about 1300.degree. F. The oxygen level in the iron will be reduced to less than 5%; however, the iron oxide particles at this point in the process are highly pyrophoric and therefore must be protected by an inert atmosphere which is provided for by the presence of nitrogen before being subjected to the second stage of the reduction step. The second stage reduction is effected at temperatures ranging from about 1600.degree. to about 1900.degree. F. and serves to completely reduce the iron oxide to iron powder. In addition to reducing the iron oxide to metallic iron, the second stage of the reduction process also serves to sinter the surface of the iron particles in such a manner so that the pyrophoricity of the particles is eliminated and therefore the iron particles may be exposed to the atmosphere with the concurrent elimination of the necessity of an inert gas such as nitrogen being present.
It is also contemplated within the scope of this invention that other steps may be employed in addition to those hereinbefore enumerated in order to insure the fact that the final silica amount in the product is less than 0.1%. For example, the iron concentrate which may contain up to about 7% silica may be subjected to a reduction step which is effected by treating the iron concentrate with hydrogen at a temperature in the range of from about 700.degree. to about 1300.degree. F. and at pressures ranging from atmospheric to about 500 psi. When utilizing this step the hematite will be reduced to magnetite. In addition to using pure hydrogen, it is also contemplated that other reducing agents such as a mixture of hydrogen and carbon monoxide may also be employed. Following the reduction of the hematite to magnetite, the iron which is present in the ore is highly magnetic and therefore the bulk of the silica contaminant is susceptible to being removed prior to caustic leaching, thus reducing the consumption of the caustic during the leaching operation. The removal of the silica gangue material from the magnetite may be accomplished by passing the ore through a permanent magnet separator. If so desired, the ore may be passed through this separator more than once; for example, the ore may be passed through the separator and recycled up to about three or four times, thus insuring a more complete separation of the magnetic iron product from the silica. The silica particles which are separated from the magnetite will tend to be the largest particles present and thus the silica which remains in the magnetite will be in the shape of rather fine particles, said fine particles being more susceptible to the caustic leach of the ore.
Following the magnetic separation of the large silica particles from the magnetite the latter may then be ground in a ball mill to a desired particle size which is suitable for the caustic leaching stage of the process. For example, the product may be ground so that the particles are present in a range of from about -100 to about +270 mesh. After grinding the ore to the desired particle size, it is then subjected to a caustic leach in a manner similar to that hereinbefore set forth whereby the soluble sodium silicates are separated from the insoluble magnetite. The separation of the iron from the soluble sodium silicates is facilitated inasmuch as magnetite leach residue filters at a rate which is more rapid than the hematite leach residue, thus permitting the separation step to be effected in a more advantageous manner. Following the separation of the solid magnetite, it is then washed with hot water at a temperature of from about 150.degree. to about 200.degree. F. to completely remove any traces of caustic leach liquor which may still be present on the ore.
Another step which may be employed to permit the recovery of a pure iron product is subjecting the magnetite solids from the filtration step, which may still contain some metal silicates such as aluminum silicate, calcium silicate, magnesium silicate, etc., as well as any sodium silicate which may have reprecipitated during the separation step, to the action of a weak acid solution. The acid wash is preferably effected at ambient temperature and atmospheric pressure for a period of time which may range from about 0.5 up to about 5 hours or more, the duration of the wash being that which is sufficient to dissolve any silica compounds which may be present. In the preferred embodiment of the invention, the acid wash comprises an aqueous hydrochloric acid solution which may contain from about 2 to about 10% hydrochloric acid, although it is also contemplated within the scope of this invention that other acids such as sulfuric acid, nitric acid, etc., may also be employed, but not necessarily with equivalent results. Upon completion of the residence time in the acid bath, the repulped solids are then filtered utilizing any conventional filtration equipment or, if so desired, the separation of the acid bath from the solids may be accomplished by other means such as decantation, etc. As in the previous step, the residue is again washed with hot water, dried and thereafter the residue is subjected to a second reduction step.
As was also previously descirbed, the reduction of the residue may be accomplished in one or more stages. The first stage is effected by treating the magnetite with pure hydrogen at a temperature in the range of from about 700.degree. to about 1300.degree. F. while the second stage is effected in the presence of hydrogen at higher temperatures ranging from about 1600.degree. to about 1900.degree. F. The result of this two stage reduction will be to lower the oxygen content of the iron powder to approximately 0.2%. Upon cooling the residue after reduction thereof, the residue will be in an agglomerated form and therefore must be subjected to a grinding step whereby the powder is reduced to a suitable particle size. The grinding of the iron to the desired particle size may be accomplished using any conventional grinding equipment such as a ball mill which will enable the operator to adjust the particle size to any mesh, said size being dependent upon the end use of the material.
Following the grinding of the iron to form the desired particle size, the powder may then be subjected to a second series of magnetic separations which will serve as a clean-up stage for any foreign particles which may still be present in the iron powder. After passage through the magnetic separator for at least one pass and preferably up to about three passes, the iron powder which is recovered will contain less than 0.1% silica. The silica compounds which remain in the iron powder in the amount hereinbefore set forth will not be discrete in form, but will be of submicron particle size and will be very finely dispersed throughout the powder. By virtue of being in submicron particle size, the finely dispersed silicon compound will not be detrimental for the use of iron powder in metallurgical applications.
By utilizing the steps hereinbefore set forth, it is possible to extract at least 99.5% of the silica with a minimal loss of iron during the process. In addition, the oxygen content of the powder will also be minimal in nature and will not interfere with the desired uses of the iron powder.